New stairs

[Redtelis]

Hi guys.

I have started to work for a project that includes a new staircase within an existing building from ground to first floor.

I never been involved with a similar project and I am starting to scheme it.

The stairs are precast so I am going to propose steel columns with landing beams to support the stairs at the corresponding levels.

What I am looking for advice is about the stability of the frame. It's literally 4 columns supporting the beam in a box shape from ground to first, no wind applied as it is internal.

I still believe that I have to provide lateral resistance even without wind applied. The first response from my colleagues is that it doesn't need to be braced or moment resisting. What do you think? I think analysed it as moment resisting is probably the best solution to avoid bracing..

 

[phamENG]

Structural stability absolutely requires some sort of lateral force resistance. For one thing, for a person to walk in one directly they have to apply some lateral force to the platform in the opposite direction (Newton and all that), and that force has to be resisted. You also have field tolerances - if one of your columns isn't perfectly plumb, or the beam is just a little shorter/longer than it was supposed to be, you'll have some inherent lateral force resolving those geometric imperfections.

Not sure how you handle it in England, but here in the states we have the Direct Analysis Method for stability design that allows you to apply "notional loads" to the structure, which amount to a small percentage of the gravity load applied laterally along with stiffness adjustments. For alternative methods, there's a catch-all that still requires some very small amount of gravity load to be applied laterally. It usually isn't much, but enough to show you that a structure with all vertical and horizontal members with nothing but pin connections won't be stable.

All that said, in a stair you can sometimes use the stair itself as a lateral brace in one direction - this is recognized in our seismic codes and requires joint releases if it's not accounted for in the modeling of seismic effects.

This doesn't address the concept of precast stairs on a steel frame. Not sure why you're going that route. I'd be interested to see some of your details when you get them sketched out - around here you use precast in concrete/masonry structures, and steel stairs in steel structures.

 

[dik]

For installation in an existing building, I would likely choose steel stairs and not precast.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[mfrad]

Agree with others that you should consider a steel stairway design rather than precast within a steel building. you should get a hold of Design Guide 34 from AISC regarding layout to get a view of most but not all the requirements for a stair. Just be aware that stairs are typically designed by architects who are responsible for a lot of items that include for tread and riser heights, finishes, slip resistance, stair widths, railing diameters, railing clearance, landing size requirements, code requirements, lighting, etc. Usually we get the geometry from the architect and check the stringer size for strength, stiffness, code live loads, vibration and floor framing supports for the stair way.

 

[XR250]

I suppose seismic is also an issue